1.Combine warm milk, water, oil and yeast in a measuring cup and stir to let yeast dissolve. Let it sit for 5 to 10 minutes to let yeast activate.
2.Add flour, sugar, baking powder and salt into the mixer bowl. Create a well and add yeast mixture into the well. Using a dough hook start mixer on low and run it for 5 minutes. Let dough rise for 1 hour or until double in size.
3.Punch middle of dough with your fist then make a few turns and reshape dough into a ball; let it rise for another hour.
4.Roll out dough to ¼” thickness and into a 4” wide and 16" long rectangle. Roll dough from the length side up. Cut dough into 16 equal pieces. If you want to add filling, roll out each piece of dough into a small disk, add a teaspoon of filling then bring the sides of the dough together and place seam side down.
5.Place buns on parchment paper squares and arrange in a steamer. Cover them with lid and let it rest for additional 30 minutes.
6.Fill steamer pot with water and bring it to a boil over high heat. Place steamer with buns into the steamer pot; make sure water is not touching buns.
7.Steam for 12-15 minutes for mantou and 15-20 minutes for filled buns until they are fluffy and fully cooked.
8.Two minutes before time is up open the lid a crack to slow air circulation. This will prevent buns from sinking right away. Serve warm.